The present invention relates to a light detector, and more particularly to a high-speed, high-sensitivity and low-noise avalanche photodiode (APD) suitable for use in optical communication.
In view of their high-speed and high-sensitivity, photodiodes (PDs) and APDs are particularly important in optical communication systems and, together with semiconductor lasers and light-emitting diodes which constitute light sources, are in process of intensive research and development.
Semiconductor lasers whose oscillation wavelength ranges from 0.7 to 1.8 microns, for instance, those having a structure of GaAlAs-GaAs or InGaAsP-InP, are of primary interest among such lasers.
As light detectors for use with GaAlAs-GaAs semiconductor lasers whose main oscillation wavelength ranges from 0.8 to 0.89 micron, PDs or APDs using crystalline silicon (Si) are extensively used, and they, supported by the advanced techniques achieved in the manufacture of Si ICs and LSIs, manifest excellent performance features, including reliability. However, a Si light detector involves the problem that the absorption coefficient of Si sharply drops at and above the wavelength of 1 micron and, in order to effectively convert light into electric signals in this wavelength region, the depletion layer thickness has to be in the order of tens of micron, which introduces a tremendous difficulty in commercial production. Especially in the wavelength range of 1.1 to 1.8 microns where optical fibers are feasible with little transmission loss, such a light detector would be of no practical use.
Meanwhile, whereas Ge-APDs or Ge-PDs are available as light detectors for use in this wavelength band of 1.1 to 1.5 microns, they have such disadvantages as a large dark current, a comparatively high level of excess noise determined by the quality of Ge, which has little room for improvement, and oversensitivity to ambient temperature variations. Therefore, a need is felt for high-quality APDs and PDs made of III-V semiconductor materials to replace the existing ones in this wavelength region of 1.1 to 1.6 microns.
An example of an APD made of such III-V semiconductor materials has been realized using InP and InGaAs. In this APA, a PN junction is formed in the InP layer and the InGaAs or InGaAsP layer is used for a light absorbing region, thereby achieving a low dark current and high multiplication. (See, for instance, Japanese Patent Publication Nos. Sho 55-132079 and Sho 56-49581.) However, if a high reverse bias is applied in order to effect quick response with this structure, the depletion layer will expand into the light-absorbing InGaAs or InGaAsP layer. As a result, the following problem has been found: The high electric field of the depletion layer expanded in the light-absorbing layer induces avalanche multiplication even in the light-absorbing layer, thereby degrading the excess noise characteristic of the InP-InGaAs of InP-InGaAsP APD structure. This is because while the ionization rate of holes in InP is greater than that of electrons, that of electrons in InGaAs or InGaAsP surpasses that of holes. This would be inconsistent with the basic concept that no low-noise APD can be obtained unless only the carrier whose ionization rate is higher is the main component of multiplication. If, conversely, the field strength in the InGaAs (or InGaAsP) layer is low, holes optically excited by reverse biasing in the InGaAs (InGaAsP) layer will move in the direction of InP having a PN junction, but they will be temporarily stored on the InGaAs-InP heterointerface on account of the valence band discontinuities in the InP-InGaAs (InGaAsP) band structure, and then either be recombined with electrons to disappear or be taken out as a slow-response component to be observed. This can be avoided by increasing the field strength on the InP-InGaAs interface, but then no low-noise APD can be obtained because of the first problem mentioned above. That is to say, the low-noise and quick-response requirements cannot be satisfied simultaneously.
Meanwhile, an attempt was recently proposed to control the ionization rates in an APD so as to make the ionization rate of electrons higher than that of holes by improving the APD structure (See, for example, F. Capasso et al, "Enhancement of Electron Impact Ionization in a Superlattice: A New Avalanche Photodiode with a Large Ionization Rate Ratio", Appl. Phys. Lett. 40(1), 1982, pp. 38-40.). This approach uses a superlattice structure made up of heterojunctions of GaAs and AlGaAs, wherein electrons generated by optical excitation receive small enough energy from an internal electric field created by reverse biasing so as not to induce an avalanche multiplication while travelling through GaAs or GaAlAs. However the generated electrons receive a quantity of energy corresponding to the discontinuities of conduction bands, attributable to the GaAlAs-GaAs heterojunction, and thereby achieve ionization. This is a remarkable new approach.